1. Ch 6: The Visual System pt 2

2. The Retina
After light passes through the pupil & lens, it hits the retina & is converted to a neural signal
Has 5 layers of different types of neurons
Receptors
Horizontal cells
Bipolar cells
Amacrine cells
Retinal ganglion cells

3. The Retina The layers of the retina are considered inside-out
Light must pass through the top 4 layers before reaching the receptor layer
Once the receptors are activated, the neural signal goes back through the layers to the retinal ganglion cells (which then exits the eyeball)

4. The Retina
There is a blind spot where the ganglion cell axons leave the eye
Fovea: an indentation at the center of the retina specialized for high acuity vision (fine details)

5. Completion
Your visual system fills in the gaps in your retinal image (created by the blind spot) by completion
Uses info from nearby receptors to “assume” what receptors in the blind spot would be “seeing” if they were there
Completion used in other scenarios
Ex: key info will be gathered about edges & the rest of the object will be filled in

6. Rods & Cones 2 types of retinal receptors Rods Cones Scotopic vision
More sensitive to light
Lacks detail & color
100s of rods converge on each retinal ganglion cell
Brain can’t “be sure” where the light is coming from
None in the fovea
Cones
Photopic vision
Dominant in good lighting
Provides hi-def color vision
A few cones converge on each retinal ganglion cell
Brain “knows” exactly where the light is coming from

7. Animal Vision

8. Eye Movement
Even though your cones are concentrated in the fovea, you can see a whole view of color
Due to constant scanning of the eyes & summation of that visual input information
Temporal integration
Essentially allows you to not notice when you blink
Your eyes continuously move and fixate on one point to the next (fixational eye movements)
Saccades

9. Eye Movement Visual neurons respond to change
If your eyes were to stop moving, your vision would fade out & stop working!!

10. Visual Transduction
Transduction: the conversion of energy from one form to another
Visual transduction: conversion of light to neural signals by the visual receptors
Rhodopsin: the red pigment in rods that absorb light
A G-protein coupled receptor that responds to light (not NTs)
Movement of Na+ ions & glutamate NTs allow for transduction in rods

11. Retina to the Brain
Main pathway is the retina-geniculate-striate pathway
Sends neural signals from each retina to the primary visual cortex (AKA striate cortex) via the lateral geniculate nuclei (LGN) of the thalamus
Called striate cortex because the cortex is layered; with stripes/striations

12. Retina to the Brain
Terminology:
Ipsilateral: same side
Contralateral: opposite side
All signals from the visual field of one side go to the primary visual cortex of the contralateral hemisphere

17. Seeing Edges Edges are the most informative features of visual stimuli
So our brains have become excellent at detecting edges
Edges are just where 2 different areas of an image meet
So our perception of an edge is a contrast between 2 adjacent areas of the visual field
Mach bands: our brains enhance the contrast at edges to make them easier to see
(we see edges as more highlighted than they are in the real world)